1. Field of the Invention
The present invention relates to prevention of backflow of resin in an injection apparatus including an injection screw with a backflow preventive ring valve at a screw tip portion provided within a heating cylinder and, particularly, a method for preventing backflow of resin effective for low-viscosity resin.
2. Description of the Related Art
In a conventional in-line screw type injection apparatus used for injection molding of resin, comprising an injection screw with a backflow preventive ring valve axially movably fitted to a screw tip portion, the injection screw being provided within a heating cylinder with a nozzle at a tip thereof so as to be rotatable and movable forward and backward, a molten resin plasticized on a feed part side is carried through a flowing gap between the rear end surface of the valve ring and a sheet ring on the circumference of a screw shaft toward the screw tip via the inside of the valve ring by rotation of the screw and metered (accumulated) in a front portion of the heating cylinder, and opening and closing of the ring valve is performed by use of the resin pressure in front and rear of the ring valve.
The opening of the ring valve is attained by the pressure of the molten resin within a screw groove moving in the valve direction from the rear of the feed part with plasticization by normal rotation (left-hand rotation) of the injection screw. The closing thereof is attained by the resin pressure generated by compression of the metered resin at the time of stopping the rotation and moving the injection screw forward to inject and fill the metered molten resin within the front portion of the heating cylinder to a mold through the nozzle.
In this closing of the ring valve, the ring valve moves with the injection screw to a position where the metered resin is sufficiently compressed by the top end surface of the screw to generate the resin pressure contributive to the valve closing. This moving stroke tends to be longer as the viscosity of the resin is lower. Therefore, the ring valve is not closed simultaneously with the forward movement of the screw, while part of the metered resin flows back within the valve ring and leaks to the screw groove until the flowing gap between the valve rear end surface and the sheet ring is closed by the resin pressure, and this leaked portion possibly affects the product weight as dispersion of metering.
As means for preventing this leak of the metered resin, it has been adapted to reversely rotate the injection screw (right-hand rotation) to reduce the resin pressure within the screw groove on the feed part side lower than the resin pressure on the metering side, move the ring valve backward to the sheet ring by this differential pressure to close it, and then move the screw forward to perform injection filling. The reverse rotation of the screw is carried out simultaneously with the start of injection or in a suck back position after the end of metering (refer to Japanese Laid-Open Patent Publication No. 62-60621 and Japanese Patent No. 2814212).
Since the above-mentioned closing of the valve by the above-mentioned reverse rotation of the screw depends on a decompression caused by the back feed of the molten resin in the screw groove by the reverse rotation, the metered resin also flows back to the feed part side until the ring valve is moved backward and closed. Accordingly, the effect remains at a reduction in metering dispersion. Further, there is a problem that since the leak quantity by backflow tends to increase as the viscosity of the resin decreases, such means are hardly adaptable to injection molding of low-viscosity resin.